subrepo: replace "os.path.exists" by "exists" via wvfs of the parent Existance of specified "path" should be examined by "exists" via wvfs of the parent repository, because the working directory of the parent repository may be in UTF-8 mode. Wide API should be used via wvfs in such case. In this patch, "/" is used as the path separator, even though "path" uses platform specific path separator (e.g. "\\" on Windows). But it is reasonable enough, because "store" and other management file handling already include such implementation, and they work well.